Alteration by natural processes or anthropogenic manipulation? Assessing human skull breakage through machine learning algorithms

Archaeological and Anthropological Sciences https://doi.org/10.1007/s12520-024-02083-5 (2024) 16:178 RESEARCH Alteration by natural processes or anthropogenic manipulation? Assessing human skull breakage through machine learning algorithms Francesc Marginedas1,2 · Abel Moclán3 · Miriam Cubas4,5 · Asier Gómez-Olivencia6,5,7 · Palmira Saladié1,2,8 · Antonio Rodríguez-Hidalgo9 Received: 2 March 2024 / Accepted: 26 September 2024 © The Author(s) 2024 Abstract Bone breakage is one of the most common features in the archaeological record. Fractures occur at different times and are classified as fresh or dry depending on the presence or absence of collagen in the bone. In the study of human remains, the timing of the occurrence of a fracture is of crucial importance as it can sometimes be linked to the cause of death. Types of skull breakage can be classified based on when they occurred, though not all fractures correspond to the expected features. This variability is added to the challenge of working with bones covered in consolidant, which obstructs the bone surface and hinders taphonomic analysis. This is the case of the Txispiri calotte, which was categorized as a skull cup in the early 20th century, though this classification was later rejected in the 1990s. In this study, we used statistics and machine learning (ML) to test the breakage characteristics of one set of skull fragments with fresh fractures, another set with dry fractures, and the Txispiri calotte. For this purpose, we considered the fracture type, trajectory, angles, cortical delamination and texture of each of the individual fractures. Our results show that the 13 fractures of the Txispiri calotte correspond to dry breakage and bear no relation to artificially produced skull cups. This study shows the potential of ML algorithms to classify fresh and dry fractures within the same specimen, a method that can be applied to other assemblages with similar characteristics. Keywords Forensic taphonomy · Skull cups · Calvaria · Dry breakage · Green breakage · Postmortem Introduction Cranial breakage is one of the most frequently studied topics in the taphonomy of human remains. These studies tend to be particularly interested in determining when bone Francesc Marginedas 1 2 Institut Català de Paleoecología Humana i Evolució Social (IPHES-CERCA), Zona Educacional 4 (Edifici W3), Campus Sescelades URV, Tarragona 43007, Spain Área de Prehistòria, Universitat Rovira i Virgili (URV), Avinguda de Catalunya 35, Tarragona 43002, Spain 3 Institute of Evolution in Africa (IDEA), University of Alcalá, Madrid, Spain 4 Área de Prehistoria. Departamento de Historia, Universidad de Alcalá, Madrid, Spain 5 Sociedad de Ciencias Aranzadi, Zorroagagaina 11, Donostia-San Sebastián 20014, Spain fractures occurred, which might be during an individual’s life (antemortem), around death (perimortem) or after death (postmortem) (Wedel & Galloway 2014). The degree of human involvement in fracture generation is one of the priorities of forensic taphonomy applied to archaeology, 6 Departamento de Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Barrio Sarriena s/n, Leioa 48940, Spain 7 Centro UCM-ISCIII de Investigación Sobre Evolución y Comportamiento Humanos, Avda. Monforte de Lemos 5 (Pabellón 14), Madrid 28029, Spain 8 Departamento de Paleobiologia, Unit Associated to CSIC, Museo Nacional de Ciencias Naturales, C/ José Gutiérrez Abascal, 2, Madrid 28006, Spain 9 Consejo Superior de Investigaciones Científicas, Instituto de Arqueología-Mérida (CSIC-Junta de Extremadura), Plaza de España 15, Mérida 06800, Spain 13 178 Page 2 of 17 where it can sometimes be associated with the cause of death (e.g., Sala et al. 2015). Humans have left traces on human skulls as the result of a multitude of practices and through a wide variety of forms and techniques, including, occasionally, their transformation into objects (Bello et al. 2011; Cid-Beziez and Pacheco-Romano 1997; TrejoMojica 2008). These skulls exhibit anthropogenic modifications such as cut marks, percussion marks, damage from blows, engravings, etc. (e.g., Gresky et al. 2017; Jeunesse 2012). In addition, this manipulation is often found linked to different contexts such as funerary treatments (Bocquentin and Garrard 2016; Esparza-Arroyo et al. 2020; Goren et al. 2001; Rivero et al. 2021; Santana et al. 2015), religious rituals (Domenici 2014; Pijoan et al. 2010), decapitations (Kanjou et al. 2015), human sacrifices (Carter 2012; Pijoan and Mansilla 1997), war trophies (Jacobi 2007; Miller 1994; Ostendorf-Smith 1995; Owsley 1994; Verhoeven 2013), surgeries (Campillo 2007), violence (Campillo 1976; Fibiger et al. 2023; Moreno-Ibáñez et al. 2021; Sala et al. 2015), and/or human cannibalism (Bello et al. 2011; Cáceres et al. 2007; Marginedas et al. 2022; Rougier et al. 2016; Sala and Conard 2016; Saladié et al. 2012; Solari et al. 2012). These forms of manipulation include the production of skull cups, objects that originated in the Upper Paleolithic and are still used today (Bello et al. 2011; Boulestin and Henry-Gambier 2019). They are made when there is still collagen in the bone (perimortem) and once all the soft tissue has been removed through controlled percussion along the cranial perimeter (outer circumference around the widest part of the head, above the ears and eyebrows), in order to separate the lower part of the skull and the face to preserve the calotte (Bello et al. 2011; Marginedas et al. 2020). This process produces a bowl shape that, according to ethnographic sources, may have been used as a drinking cup (Davis 1867; Hocart et al. 1993; Massola 1961; Meehan 1971). At European prehistoric sites, skull cups are primarily found in association with cannibalism events (Bello et al. 2011; Boulestin and Coupey 2015; Boulestin and HenryGambier 2019; García-Sanchez and Carrasco-Rus 1981; Jiménez Brobeil 1990; Marginedas et al. 2020; Saladié et al. 2015; Santana et al. 2019). Different uses have been proposed for these prehistoric artifacts based on their shape and context, including drinking vessels, family relics or even war trophies (Bello et al. 2011; Boulestin 2012). One of the first researchers to link the presence of human calottes in the archaeological record to intentional manipulation in cannibalistic contexts was Weidenreich (1944, 1951). Based on bone breakage, Weidenreich suggested that the skull shapes of Zhoukoutien (Middle Pleistocene; China) were the result of headhunting or war trophies, as well as corpse consumption. However, these studies were dismissed more recently by Boaz and colleagues (2000), 13 Archaeological and Anthropological Sciences (2024) 16:178 who related skull breakage and other modifications on the bone surface to carnivore activity. Similar cases were documented at Modjokerto (Indonesia) (Jacob 1964) and Makapansgat (South Africa) (Dart 1962). These interp (...truncated)